We're using SQL for our file database, so such behavior would be something to be addressed best to the developers of the SQL solution (sqlite) we're currently using, however I really doubt this quadratic behavior your're suggesting... I do have a quite big library myself and so when rehashing, I should have encountered the same - especially as I did pay attention to the speed files were added at the time.
mfg,
Old
PS.: I've got no clue about who's bee removing your posts.
[EDIT]
PPS.: Concerning your other (removed) post: I do agree doing calculations in the same thread as handling the GUI is not a very good approach, but unless there's somebody willing to do a tremendous amount of coding for Shareaza, we're stuck with what we have inherited from the past, and there's no way to change that. However as you seem to know quite well about what you're talking, I do invite you to have a look at our source code. Maybe you could post some optimizations to Shareaza yourself. (At least I for once think that investing the time in coding is better then using it up in forums )